The present invention generally relates to a developing method for use in electrography and more particularly, to a magnetic brush developing method for use in electrography which utilizes a two or dual-component developing material including electrically insulative magnetizable particles as carrier and electrically insulative non-magnetizable particles as toner.
Recently, for a developing material to be used for magnetic brush development in electrography, electrically insulative magnetizable particles of small diameter (5.about.30.mu.) prepared by dispersing fine magnetizable powder into a bonding agent or bonding material have come to be practically employed as carrier instead of the conventional iron particles, etc. of larger diameter (80.about.200.mu.). In connection with the above, a developing material composed of electrically insulative magnetizable particles with average particle diameter of 5 to 30.mu. prepared by dispersing magnetizable powder into electrically insulative resin having specific resistance higher than 10.sup.14 .OMEGA..cm, and electrically insulative non-magnetizable particles (toner) with average particle diameter of 3 to 30.mu. and volume resistivity higher than 10.sup.14 .OMEGA..cm, has conventionally been put into actual use as disclosed, for example, in Japanese Laid Open Patent Application Tokkaisho 54-66134. The known dual-component developing material of the above described kind has advantages in that the quality of the copied images is superior, and moreover, the range of allowance for variations of mixing ratio of carrier to toner is large as compared with the conventional developing materials. On the other hand, however, in the conventional dual-component developing material of the above described type, there have been such problems that, since the electrically insulative magnetizable particles as carrier are very small in size, they tend to adhere to a photoreceptor surface during development. When the developing material remaining on the photoreceptor surface is removed by a blade cleaner or the like after transfer of developed images, the magnetizable particles are held between the photoreceptor surface and the blade. This results in various inconveniences such that the magnetizable powder contained in the magnetizable particles injures the photoreceptor surface or that, since the magnetizable particles are gradually consumed in a very small amount due to their adhesion to the photoreceptor surface, the amount of carrier tends to be decreased during continuous use for a long period of time unless such magnetizable particles are collected for re-use, consequently giving rise to insufficient development due to shortage of carrier.